The probing of high frequency signals, whether for an Oscilloscope or a Logic Analyzer, presents certain technical problems that increase in difficulty as the frequency of the signal gets higher. Quite aside from attaining the requisite bandwidth in the ‘Scope or Logic Analyzer, careful attention has to be paid to the way the signals of interest (the ‘target signals’) are acquired through suitable probes. At the highest frequencies, say, above ten GHz for ‘Scopes and above about a one GHz clock rate for Logic Analyzers, probe related issues that could be often be ignored at lower frequencies emerge as factors that limit overall system performance.
The incorporated ‘SIGNAL PROBE AND PROBE ASSEMBLY’ is a way to deal with at least one of these issues: getting a necessary resistor in the probe's tip as close as possible to the target signal. Probes for different kinds of test equipment (e.g., ‘Scopes, Logic Analyzers, Spectrum Analyzers, etc.) have such tip resistors for different reasons, such as damping, minimizing loading, attenuation, or, perhaps impedance matching. What follows the probe tip is generally a length of coaxial cable having a particular characteristic impedance (Z0) that should not be thought of as merely a shielded conductor, but as a transmission line. Generally there is an active circuit of some sort at the other end of the coax, such as an amplifier with a particular frequency response, or, a threshold detector. The active circuit's output is then sent by a robust buffer amplifier over suitable transmission lines to the particular measurement circuitry in use. Probe architectures vary, but in each case it remains desirable to get the tip resistor as close as possible to the target signal.
There are two general reasons for this. First, any extra distance represents length along a conductor that will exhibit both a parasitic series inductance and a parasitic shunt capacitance to any nearby AC ground. These parasitic reactances represent target signal loading over an above what the probe's coupling network proper is intended to present. These parasitic reactances can be the cause of reflections and bandwidth limitations. Second, even if those parasitic reactances do not noticeably cause reflections and reduced bandwidth, they still amount to an unwanted intervening impedance that means that the signal presented to the entrance of the probe tip's resistor is not the identically same as the one contacted by the very end of the probe's tip. This is an issue of signal fidelity within the probe, quite apart from, and even in the absence of, significant loading.
The spring pin disclosed in ‘SIGNAL PROBE AND PROBE ASSEMBLY’ reduces the length of the probe tip by placing the probe tip resistor very close to the mechanical location of electrical contact with the target signal. It is almost as if one end of that resistor were the actual mechanical point of contact. We can't really achieve exactly that, because the resistor is an SMT part with rather flat surfaces for leads, while the contacting surface for the probe needs be a durable needle point or some non-slip variation on that, such as a ‘crown point’ having a plurality of points around a central depression. The next best thing is for the target side of the tip resistor to carry the durable point, while the other end of the resistor is mounted in a socket that in turn is carried by the larger contact structure and is mechanically biased against axial movement by a spring. ‘SIGNAL PROBE AND PROBE ASSEMBLY’ goes on to show an inline array of such spring pins carried by a PCB (Printed Circuit Board) that couples each of those spring pins to an associated coaxial cable that is a transmission line of characteristic impedance Z0 and that leads to some type of test equipment. We shall hereinafter call such a spring pin contact a ‘resistor tip spring pin’ contact.
As significant an improvement as ‘SIGNAL PROBE AND PROBE ASSEMBLY’ represents, there is still an aspect of its operation that can be improved. In particular, and as will become apparent when the Drawings are considered, the region of the spring pin: (a) which telescopes in and out of the housing containing the spring; and (b) which bears the socket that receives the tip resistor; (c) represents a variable length (depending upon how far down the entire probe assembly is moved to ensure good contact for all the probe tips carried by the assembly) that also has an arbitrary characteristic impedance (Zx) that in general is not equal to that (Z0) of the interconnecting cables. This mischief arises owing to the placement of the aforementioned housing right at the edge of the PCB (in the belief that this would maximize the variation in the amounts that different spring pins may move when contact is established). That is, the pin driven by the spring and carrying the tip resistor is sticking out into space by some unregulated amount, amounting to a transmission line of arbitrary Zx. Once the pin enters the housing, however, the characteristic impedance is controlled (Z0), and remains so from there on as the signal proceeds further into the probe assembly. At high frequencies, the transition from the short length of uncontrolled Zx into the controlled Z0 is a discontinuity that manifests itself as reflections in the time domain and as frequency response anomalies in the frequency domain. It would be advantageous to adjust Zx to be the same as Z0. What to do?